Communication system and communication terminal

ABSTRACT

A transmitting circuit is connected to a communication line and configured to transmit a primary signal, being a current signal, to the communication line by changing a current flowing from a transmission unit via the communication line. A receiving circuit is configured to receive a secondary signal which is generated by converting the primary signal transmitted from the transmitting circuit to a voltage signal by a current/voltage converter provided between the transmission unit and the communication line. Specifically, the primary signal, being a current signal transmitted from a communication terminal, is converted to the secondary signal being a voltage signal by the current/voltage converter. As a result, communication can be performed even when the impedance of the communication line is reduced, facilitating introduction of a communication system.

TECHNICAL FIELD

The present invention relates to a communication system including aplurality of communication terminals connected to a two-wire typecommunication line, and a power supply device that applies voltagebetween two wires of the communication line, and relates to acommunication terminal.

BACKGROUND ART

Conventionally known communication system is that abase unit and aplurality of communication terminals (subbase units) are connected to acommunication line (transmission line), to thereby perform communicationbetween each communication terminal and the base unit. As an example ofthis kind of communication system, there is a system in which the baseunit regularly monitors a state of the communication terminal, and whenthere is a change in the state of the communication terminal, a signalis transmitted from the base unit to other communication terminal so asto respond to the change of the state (see Japanese Patent PublicationNo. 1180690, Japanese Patent Publication No. 1195362, and JapanesePatent Publication No. 1144477). In this kind of communication system,for example when on/off state of a switch appended to the communicationterminal is changed as the state change of the communication terminal,the base unit turns on/off an illumination apparatus and an airconditioner connected to other communication terminal, as a processingfor responding to the state change. In this communication system, asshown in FIG. 25, communication terminals 1 don't perform communicationdirectly but always perform communication via a base unit 2.

However, in the above-described structure, the communication terminals 1always perform communication via the base unit 2, and therefore acommunication speed is slow for performing polling by the base unit 2directed to the communication terminals 1. Accordingly, thecommunication system with this structure is unsuitable for transmittinginformation with relatively large volume of data such as analog data(continuous data of electric energy). Further, in the communicationsystem with this structure, an overall system is halted at the time of afailure, etc., of the base unit 2, thus involving a problem thatreliability is low.

Meanwhile, there is also proposed a communication system configured todirectly perform communication by communication terminals connected to acommunication line in a peer-to-peer mode (called P2P hereafter),capable of transmitting the information with relatively large volume ofdata by improving the communication speed. Note that in thiscommunication system, power is supplied to a plurality of communicationterminals by one power feeder connected to the communication line.

The communication system of directly performing communication withcommunication terminals as described in the latter communication system,is desirable from a viewpoint of the communication speed andreliability. However, the former communication system in whichcommunication is performed by the communication terminals 1 via the baseunit 2 is widely spread. Therefore, in order to effectively utilize analready established communication system, the communication system asshown in FIG. 26 can be considered, which is the communication system ofa coexistence of the former communication system, and the lattercommunication system capable of performing high-speed communication (seeJapanese Patent Application Laid-Open No. H8-274742)

In the communication system of FIG. 26, a first communication terminal 1performs communication via the base unit 2 by using a first protocolsignal (voltage signal). Meanwhile, a second communication terminal 10performs direct communication by using a second protocol signal (voltagesignal) which is superimposed on the first protocol signal in a mannerof synchronizing with the first protocol signal. The second protocolsignal has a higher frequency than the frequency of the first protocolsignal, and there is a difference in a signal level, etc., between thefirst protocol signal and the second protocol signal. Therefore, thefirst communication terminal 1 and the second communication terminal 10cannot perform communication with each other, although they areconnected to the same communication line 4.

Incidentally, in the aforementioned communication system, a plurality ofterminals (base unit 2, first communication terminal 1, and secondcommunication terminal 10) are connected to the two-wire typecommunication line 4. Therefore, an input impedance of each terminal isconnected in parallel via the communication line 4. Accordingly, inorder to perform communication by using the voltage signal (secondprotocol signal) between second communication terminals 10, theimpedance of the terminal connected to the communication line 4 isrequired to be increased. However, the terminal with low impedance alsoexists in the already established terminals. Particularly, the base unit2 also functions as a power supply and includes a smoothing capacitor inan output stage, thus showing relatively low impedance. Such a terminalis required to have high input impedance to a signal component, byconnecting a high impedance module (not shown) between the terminal andthe communication line 4, to thereby provide high impedance to thesecond protocol signal.

However, when the already established communication system is utilized,a contractor needs to connect the high impedance module between theterminal with low impedance of the aforementioned already establishedterminals, and the communication line 4, to thereby performcommunication between the second communication terminals 10 by using thevoltage signal. Therefore, when the second communication terminals 10are introduced, the contractor needs to confirm a position of thealready established terminal to perform a work of connecting the highimpedance module. If the contractor forgets to connect the highimpedance module, there is a possibility that the impedance of thecommunication line 4 is reduced, and the communication between thesecond communication terminals 10 is not established.

As described above, there is a problem in introducing the aforementionedcommunication system, such that it is time-consuming and laborious toexamine a situation of a work site, thus imposing a large load on thecontractor, and therefore the aforementioned communication system cannotbe easily introduced.

SUMMARY OF THE INVENTION

In view of the above-described circumstance, the present invention isprovided, and an object of the present invention is to provide acommunication system and a communication terminal capable of performingcommunication even in a case of low impedance of a communication line,thereby facilitating introduction of this communication system.

A communication system of the present invention includes: a plurality ofcommunication terminals connected to a two-wire communication line; apower supply device configured to apply voltage between two wires of thecommunication line; and a current/voltage converter configured toconvert change of current on the communication line to change of voltageon the communication line by a voltage drop by a resistance component,the communication terminals each including: a transmitting circuitconnected to the communication line and configured to generate a primarysignal, being a current signal, on the communication line by changing acurrent flowing from the communication line; and a receiving circuitconfigured to receive a secondary signal which is generated byconverting the primary signal to a voltage signal by the current/voltageconverter.

According to the present invention, the signal from the transmittingcircuit of the communication terminal is transmitted as the currentsignal, and the signal is received by the receiving circuit of thecommunication terminal as the voltage signal which is obtained byconverting the current signal to the voltage signal by thecurrent/voltage converter. Therefore, there is an advantage thatintroduction of the communication system is facilitated.

In this communication system, preferably characteristic impedance of thecommunication line is utilized as the resistance component by thecurrent/voltage converter.

In this communication system, preferably a resistance element connectedbetween the power supply device and the communication line is utilizedas the resistance component by the current/voltage converter.

In this communication system, preferably an inductance element connectedbetween the power supply device and the communication line is utilizedas the resistance component by the current/voltage converter.

In this communication system, preferably the receiving circuit receives,as the secondary signal, the change of voltage which is generated on thecommunication line due to voltage drop when the current flowing from thecommunication line is changed by the transmitting circuit.

In this communication system, preferably the power supply deviceincludes a signal transmitting section that transmits a first protocolsignal, being a voltage signal, to the communication line by changing amagnitude of a voltage applied to the communication line, and thereceiving circuit receives, as the secondary signal, a secondaryprotocol signal having a higher frequency than a frequency of the firstprotocol signal and superimposed on the first protocol signal.

In this communication system, further preferably the receiving circuitincludes a comparator circuit that extracts a signal component of thesecondary signal by binarizing the secondary signal by comparing themagnitude of the secondary signal with a predetermined threshold value,and when transmitting the primary signal from the transmitting circuit,the communication terminal sets input impedance viewed from thecommunication line to be low to suppress an amplitude of the secondarysignal so that a component excluding the signal component of thesecondary signal does not exceed the threshold value.

In this communication system, further preferably the receiving circuitincludes an amplifier circuit configured to perform conversion to changeof voltage from a first reference voltage by amplifying the secondarysignal; and a comparator circuit configured to binarize the secondarysignal by comparing an output of the amplifier circuit with a secondreference voltage, the first reference voltage being set to be deviatedfrom the second reference voltage.

In this communication system, further preferably the receiving circuitincludes a comparator circuit configured to binarize the secondarysignal by comparing a magnitude of the secondary signal with apredetermined threshold value; and a correction circuit configured todetect rising or falling of the secondary signal as a trigger, andcorrect the secondary signal to a square wave having a predeterminedpulse width from a detection point of the trigger.

In this communication system, preferably the transmitting circuitincludes an encoding section configured to encode transmission data suchthat a value not allowing the trigger to be generated is not continuedin the secondary signal, and the receiving circuit includes a decodingsection configured to decode the encoded data obtained from thesecondary signal to acquire the transmission data.

The communication terminal of the present invention is used in acommunication system that includes a power supply device configured toapply voltage between two wires of a communication line, and acurrent/voltage converter configured to convert change of current on thecommunication line to change of voltage on the communication line by avoltage drop by a resistance component, the communication terminalincluding a transmitting circuit connected to the communication line andconfigured to transmit a primary signal, being a current signal, to thecommunication line by changing a current flowing from the communicationline; and a receiving circuit configured to receive a secondary signalwhich is generated by converting the primary signal to a voltage signalby the current/voltage converter.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic system block diagram showing a structure ofEmbodiment 1.

FIG. 2 is an explanatory view of a transmission signal format used inthis system.

FIG. 3 is a schematic circuit view showing an essential part of thissystem.

FIG. 4 is a waveform chart showing an operation of a transmittingcircuit of this system.

FIG. 5 is a schematic circuit view showing a structure of a receivingcircuit of this system.

FIG. 6 is a waveform chart showing the operation of the receivingcircuit of this system.

FIG. 7 is a schematic system block diagram showing a positional relationof a communication terminal of this system.

FIG. 8 is a graph showing amplitude of a received signal of this system.

FIG. 9 is a graph showing the amplitude of the received signal of thissystem.

FIG. 10 is a waveform chart showing the operation of a comparativeexample of this system.

FIG. 11 is a waveform chart showing the operation of this system.

FIG. 12 is a waveform chart showing the operation of the comparativeexample of this system.

FIG. 13 is a waveform chart showing the operation of other comparativeexample of this system.

FIG. 14 is a waveform chart showing the operation of this system.

FIG. 15 is an explanatory view of the operation of Embodiment 2.

FIG. 16 is an explanatory view of the operation of this system.

FIG. 17 is an explanatory view of the operation of this system.

FIG. 18 is an explanatory view of the operation of this system.

FIG. 19 is an explanatory view of the operation of the comparativeexample of this system.

FIG. 20 is a schematic block diagram showing other example of acurrent/voltage converter.

FIG. 21 is a waveform chart showing an operation example of thereceiving circuit in embodiments 1 and 2.

FIG. 22 is a waveform chart showing the operation example of thereceiving circuit in embodiments 1 and 2.

FIG. 23 is a waveform chart showing the operation example of thereceiving circuit in embodiments 1 and 2.

FIG. 24 is a waveform chart showing the operation example of thereceiving circuit in embodiments 1 and 2.

FIG. 25 is a schematic system block diagram showing a conventionalexample.

FIG. 26 is a schematic system block diagram showing other conventionalexample.

BEST MODE FOR CARRYING OUT THE INVENTION

First, a basic structure of a communication system according to eachembodiment will be described below. The communication system of thefollowing embodiment is the communication system comprising atransmission unit (base unit) 2 connected to a two-wire typecommunication line 4 (see FIG. 26). For example a system such as NMAST(registered trademark) is given for example as this kind ofcommunication system. This communication system comprises a plurality of(two in an example of the figure) first communication terminals 1, 1connected to the communication line 4 and configured to communicate withthe transmission unit 2; and a plurality of (two in the example of thefigure) second communication terminals 10, 10 connected to thecommunication line 4 and configured to directly communicate with eachother. In this communication system, communication is performed by usinga first protocol signal transmitted through the communication line 4,and a second protocol signal superimposed on the first protocol signal.The second protocol signal has a higher frequency than the frequency ofthe first protocol signal.

A plurality of first communication terminals 1, 1 are connected inparallel to the transmission unit 2 by the communication line 4. Atime-sharing multiplexing transmission system (called a “basic system”hereafter) is constructed by the transmission unit 2 and the firstcommunication terminals 1, wherein data transmission to thecommunication terminals 1 from the transmission unit 2, and datatransmission to the transmission unit 2 from the first communicationterminals 1 are carried out by time-sharing.

In the basic system, the first communication terminals 1 are classifiedinto two kinds of a monitoring terminal unit provided with a switch anda sensor, etc., (not shown), and a control terminal unit provided with aload (not shown). Thus, the load provided to the control terminal unitcan be controlled according to first monitoring information from theswitch and the sensor provided to the monitoring terminal unit. Anaddress (identifier) is respectively assigned to the first communicationterminal 1. When the first monitoring information is received, themonitoring terminal unit transmits control information corresponding tothe first monitoring information, to the transmission unit 2. When thecontrol information is received, the transmission unit 2 transmits thecontrol information to the control terminal unit corresponding to themonitoring terminal unit based on the address. When the controlinformation is received, the control terminal unit controls the loadaccording to the control information. Since the control information forcontrolling the load reflects the first monitoring information, thefirst monitoring information is reflected on the control of the load bytransmitting the control information through the communication line 4,although the transmission unit 2 is interposed between the monitoringterminal unit and the control terminal unit.

Subsequently, an operation of the basic system will be described.

The transmission unit 2 transmits to the communication line 4, atransmission signal (first protocol signal) having a voltage waveform ofa form as shown in FIG. 2. Namely, the transmission signal is a bipolar(±24 V) time-sharing multiplexing signal composed of an interruptionpulse period 41, a spare period 42, a signal transmitting period 43, asignal returning period 44, an interruption pulse period 45, ashort-circuit detecting period 46, and a spare region period 47. Theinterruption pulse period 41 is a period for detecting an interruptionsignal as will be described later, the spare period 42 is a period setcorresponding to the interruption pulse period 45 and the short-circuitdetecting period 46, and the signal transmitting period 43 is a periodfor transmitting data to the first communication terminal 1. The signalreturning period 44 is a time slot for receiving a return signal fromthe first communication terminal 1, the interruption pulse period 45 isa period for detecting the interruption signal, and the short-circuitdetecting period 46 is a period for detecting short-circuit. The spareregion period 47 is a period for a case that processing cannot becarried out in time. The transmitting signal is a signal fortransmitting data by modulating a carrier composed of a pulse train, bypulse width.

In each first communication terminal 1, when address data included inthe signal transmission period 43 of the transmission signal receivedvia the communication line 4, coincides with the address assigned toeach first communication terminal 1, the control information forcontrolling the load is captured from the transmission signal. At thistime, the first communication terminal 1 further returns the controlinformation synchronously to the signal returning period 44 of thetransmission signal, as a current mode signal (signal transmitted byshort-circuiting the communication line 4 via a proper low impedance).Further, power of an internal circuit of the first communicationterminal 1 is supplied by rectifying and stabilizing the transmissionsignal transmitted via the communication line 4.

Namely, the transmission unit 2 carries out regular polling bysequentially accessing the first communication terminals 1, 1, withaddress data changed cyclically, which is usually included in thetransmission signal. During such a regular polling, if the controlinformation is included in the transmission signal, the firstcommunication terminal 1 with its self-address matched with the addressdata included in the transmission signal, is operated after capturingthe control information, and returns a self-operation state to thetransmission unit 2.

Meanwhile, when an interruption signal is received, which is generatedin any one of the monitoring terminal units (first communicationterminals 1) corresponding to the first monitoring information, thetransmission unit 2 carries out interruption polling as well. In thisinterruption polling, the transmission unit 2 searches the firstcommunication terminals 1 which generate the interruption signal, andthereafter accesses the first communication terminals 1, so that thecontrol information responding to the first monitoring information isreturned.

Namely, the transmission unit 2 carries out regular polling in which thetransmission signal with address data changed cyclically is usuallytransmitted to the communication line 4. Further, when the interruptionsignal generated in the monitoring terminal units (first communicationterminals 1) is detected synchronously to the interruption pulse period41 or the interruption pulse period 45 of the transmission signal, thetransmission unit 2 transmits the transmission signal having mode dataset in an interruption polling mode. When a high-order bit of theaddress data of the transmission signal set in the interruption pollingmode, coincides with the high-order bit of its self-address, the firstcommunication terminal 1 that generates the interruption signal returnsa low-order bit of its self-address as return data synchronously to thesignal returning period 44 of the transmission signal. Thus, the addressof the first communication terminal 1 that generates the interruptionsignal can be acquired by the transmission unit 2.

When the address of the first communication terminal 1 that generatesthe interruption signal, is acquired by the transmission unit 2, thetransmission unit 2 transmits the transmission signal for requestingreturn of the control information, to the first communication terminal1. Then the first communication terminal 1 returns the controlinformation corresponding to the first monitoring information, to thetransmission unit 2. When the control information is received, thetransmission unit 2 gives an instruction to clear the first monitoringinformation of the corresponding first communication terminal 1, and thefirst monitoring information is cleared by the first communicationterminal 1.

The transmission unit 2 that receives the control information, generatesthe control information transmitted to the first communication terminal(control terminal unit) 1 which is associated with the firstcommunication terminal (monitoring terminal unit) 1, being an origin ofthis control information, based on the correlation of the address. Thetransmission unit 2 transmits the transmission signal including thiscontrol information to the communication line 4, and controls the loadprovided to the first communication terminals (control terminal units)1.

In the aforementioned basic system, the first communication terminals(monitoring terminal unit, control terminal unit) 1 performcommunication with, each other via the transmission unit 2, according toa protocol of a polling selecting system (called a first protocolhereafter).

Incidentally, in the aforementioned system, a plurality of secondcommunication terminals 10, 10 are connected in parallel to each othervia the communication line 4 so as to share the communication line 4with the aforementioned basic system. Monitored equipment (not shown)for outputting second monitoring information transmitted between thesecond communication terminals 10, is connected to one of the secondcommunication terminals 10, and a monitoring device (not shown) foracquiring the second monitoring information from the secondcommunication terminal 10, is connected to the other secondcommunication terminal 10.

Namely, the second communication terminal 10 performs communication(data transmission) via the communication line 4, and meanwhile themonitored equipment generates transmission data (second monitoringinformation), and the monitoring device processes received data. Thesecond communication terminal 10 functions as an adaptor for convertingdata from the monitored equipment or the monitoring device connected toeach second communication terminal 10, and transmitting it to thecommunication line 4. The monitored equipment and the monitoring devicereceive and transmit data from/to the second communication terminal 10by regularly performing communication with each other. Note that a powermeasuring instrument for measuring power consumption of lightingequipment controlled by the basic system, can be considered as anexample of the monitored equipment, and a meter reading device fordisplaying the power consumption measured by the power measuringinstrument can be considered as an example of the monitoring device.

The second communication terminal 10 has a function of transmitting data(second monitoring information) to other second communication terminal10 not through the transmission unit 2, based on a protocol differentfrom the aforementioned first protocol (called a second protocolhereafter). There is a difference in the frequency and a signal level,etc., between the first protocol signal and the second protocol signal,and therefore although the first communication terminal 1 and the secondcommunication terminal 10 are connected to the same communication line 4and can share the communication line 4, they cannot performcommunication with each other.

Specifically, the second communication terminal 10 superimposes a packeton the transmitting signal based on the second protocol, the packetincluding data to be transmitted to other second communication terminal10, and transmits it to the communication line 4, and receives a secondprotocol packet transmitted by other second communication terminal 10.Namely, the first communication terminals 1 perform communication witheach other based on the first protocol via the transmission unit 2 asdescribed above, and meanwhile the second communication terminals 10directly perform communication with each other based on the secondprotocol, and don't depend on the transmission unit 2. Therefore, thecommunication speed of the communication based on the second protocolcan be faster than the communication speed of the communication based onthe first protocol, and such a communication is used for thetransmission of information with relatively large volume of data likeanalog data (continuous data of electric energy).

Further, the second communication terminal 10 monitors the transmissionsignal of the first protocol transmitted between the transmission unit 2and the first communication terminal 1 of the basic system, and analyzesa data transmission state of the first protocol (called a statehereafter) from the monitored transmission signal. Further, the secondcommunication terminal 10 has a function of judging whether or not thestate is suitable for transmitting the second protocol packet, andtransmitting the packet at a time when it is judged to be suitable fortransmission.

Namely, the transmission signal obtained by performing pulse-widthmodulation of the carrier, the carrier being composed of a pulse train,is used in the first protocol which is used in the basic system. Forsuperimposing the second protocol packet on this transmission signal, itis preferably superimposed on the transmission signal in a period whenthe transmission signal is stable in a high-level or in a low-level.

Here, the transmission signal employs a signal format as shown in FIG.2. Therefore, although the spare period 42, the short-circuit detectingperiod 46, and the spare region period 47 can be considered to be theperiod suitable for transmitting the packet (called a communicationsuitable period hereafter), the other periods can be considered to bethe periods not suitable for transmitting the packet (called acommunication unsuitable period hereafter). Namely, the spare period 42,the short-circuit detecting period 46, and the spare region period 47are suitable for transmitting the packet, because the time in which thetransmission signal is stable in the high level or in the low level isrelatively long. Other periods are unsuitable for transmitting thepacket, because the time in which the transmission signal is stable inthe high-level or in the low-level is relatively short, and because thepacket is easily affected by the transmission of the signal based on thefirst protocol (interruption signal and return data) between thetransmission unit 2 and the first communication terminal 1. Further,rising and falling periods of the transmission signal can also beregarded as the communication unsuitable periods because of an influenceof a harmonic noise and an influence of a transient response due tovoltage inversion of the signal.

Therefore, the second communication terminal 10 is configured to analyzethe state of the transmission signal, judge whether the state is thecommunication suitable period or the communication unsuitable periodbased on its analysis result (state of the transmission signal), andtransmit the second protocol packet only when it is so judged that thestate is the communication suitable period. Thus, by superimposing thesecond protocol packet on the transmission signal synchronously to thefirst protocol transmission signal, interference between thecommunication of the first protocol and the communication of the secondprotocol using the common communication line 4, can be prevented. Whendata cannot be completely transmitted in one communication suitableperiod due to large volume of data of the transmission data, the secondcommunication terminal 10 suspends the communication at the end of thecommunication suitable period, and transmits remained data in the nextcommunication suitable period.

Note that power can be supplied to each section of the secondcommunication terminal 10 by a method of supplying power by rectifyingand stabilizing the transmission signal transmitted via thecommunication line 4 from the transmission unit 2 similarly to themethod in the first communication terminal 1 of the basic system(intensive feeding method). However, the method of supplying power toeach section of the second communication terminal 10 is not limited inthis configuration, and the power may be supplied by a method byrectifying and stabilizing a commercial power supply (local feedingmethod).

Embodiment 1

The communication system of this embodiment is characterized in thecommunication between the second communication terminals 10, 10 in theaforementioned communication system. The structure and the function ofthe first communication terminal 1 are based on the basic structuredescribed above, and therefore explanation for the first communicationterminal 1 is omitted hereafter, and the second communication terminal10 is simply expressed as “communication terminal 10” hereafter in theexplanation.

In the communication system of this embodiment, as shown in FIG. 1, acurrent/voltage converter 3 for converting change of current on thecommunication line 4 to change of voltage on the communication line 4,is provided between the transmission unit 2 and the communication line4. The transmission unit 2 functions as a power supply device forapplying voltage (transmission signal here) between two wires of thecommunication line 4.

The communication terminal 10 includes a rectifier 11 composed of adiode bridge for rectifying the transmission signal transmitted from thetransmission unit 2 via the communication line 4, and a power supplycircuit 13 connected to an output of the rectifier 11 via an impedancecircuit 12, and generates an internal power supply by the power supplycircuit 13. Further, the communication terminal 10 includes atransmitting circuit 14 for transmitting the second protocol signal, areceiving circuit 15 for receiving the second protocol signal, anequipment communicating section 16 for performing communication with themonitored equipment and the monitoring device, and a controller 17 forcontrolling an operation of each section. Note that the rectifier 11 isused for supplying power, and also eliminates polarities of the signal.

The controller 17 is mainly composed of a microcomputer, and outputs asquare wave transmission signal to the transmitting circuit 14, andinputs a square wave received signal from the receiving circuit 15. Thecontroller 17 has a function of transmitting data (second monitoringinformation) acquired by the equipment communicating section 16 from themonitored equipment, to other communication terminal 10 from thetransmitting circuit 14, and outputting the data acquired by thereceiving circuit 15 from other communication terminal 10, to themonitoring device from the equipment communicating section 16.

The transmitting circuit 14 is connected to the communication line 4 andis configured to transmit a primary signal 51, being a current signal,to the communication line 4, by changing the current which flows fromthe communication line 4. Further, the receiving circuit 15 isconfigured to receive a secondary signal 52, being a signal obtained byconverting the primary signal 51 transmitted from the transmittingcircuit 14 to a voltage signal by the current/voltage converter 3.Namely, the primary signal 51, being the current signal, transmittedfrom the communication terminal 10 is converted to the secondary signal52, being the voltage signal, by the current/voltage converter 3.

Next, specific structures of the current/voltage converter 3 and thetransmitting circuit 14 will be described, with reference to FIG. 3.

The current/voltage converter 3 is composed of resistance elements R11,R12 inserted between an output end of the transmission unit 2 and thecommunication line 4. The resistance elements R11, R12 may be insertedwith respect to at least one wire of the two-wire type communicationline 4. In this embodiment, they are inserted for two wires together,and resistance values of both resistance elements R11, R12 are set tosame values. With this structure, the current/voltage converter 3converts change of current on the communication line 4 to change ofvoltage on the communication line 4 by voltage drop by the resistanceelements R11, R12. Accordingly, the amplitude of the secondary signal 52is changed by resistance values of the resistance elements R11, R12connected between the transmission unit 2 and the communication line 4.Therefore, an adjustment of improving a transmission quality can becarried out by changing a setting of the resistance values of theresistance elements R11, R12.

However, actually the characteristic impedance of the communication line4 itself also plays an important role as a resistance component ofcurrent/voltage converter for converting change of current on thecommunication line 4 to change of voltage on the communication line 4.Namely, when the primary signal 51, being the current signal, istransmitted to the communication line 4 by the communication terminal10, the resistance component including the communication line 4 itselfthat exists between the communication terminal 10 and the transmissionunit 2, being the power supply device, functions as the current/voltageconverter, to thereby convert the primary signal 51 to the secondarysignal 52. Further, not only the characteristic impedance of thecommunication line 4, but also all of the resistance components thatexist between the transmitting side communication terminal 10 and thepower supply device, such as output impedance of the transmission unit2, can be used as the current/voltage converter.

Therefore, even when the resistance component such as characteristicimpedance of the communication line 4 and output impedance of thetransmission unit 2 is used as the current/voltage converter withoutnewly providing the resistance elements R11, R12, the communicationsystem with the same operation as this embodiment can be realized.Namely, the current/voltage converter utilizes the characteristicimpedance of the communication line 4 and the output impedance of thetransmission unit 2, etc., as the resistance component, there is nonecessity for newly adding the current/voltage converter on thecommunication line 4, thus making it possible to suppress theconstitutional elements of the communication system to be small.

Thus, in consideration of the characteristic impedance of thecommunication line 4, etc., even when the resistance elements R11, R12are provided, the resistance values of the resistance elements R11, R12can be sufficiently small (for example, 4.7Ω). In this case, theresistance elements R11, R12 function as an impedance regulating sectionfor regulating an impedance of the transmission unit 2 viewed from theconnection line 4. However, when the characteristic impedance of thecommunication line 4 is used as the current/voltage converter asdescribed above, a distributed constant circuit needs to be taken intoconsideration, thus complicating the explanation. Therefore, in order tosimplify the explanation hereafter, the system is considered as a lumpedconstant circuit with resistance elements R11, R12 used as thecurrent/voltage converter 3.

As shown in FIG. 3, the transmitting circuit 14 includes a filtercircuit 18 connected to an input end 101 and configured to convert asquare wave transmission signal (voltage signal) input from the inputend 101, to a sinusoidal voltage signal, and includes a current controlcircuit 19 connected to an output of the filter circuit 18. The inputend 101 is connected to the controller 17.

In an example of FIG. 3, the filter circuit 18 is an active filter usingan operational amplifier OP1 in which a reference voltage is applied tothe non-inverting input terminal. The filter circuit 18 includesresistance R1, resistance R2, and resistance R3 sequentially connectedin series from input end 101 side, between the input end 101 and theinverting input terminal of the operational amplifier OP1, and acapacitor C1 with one end connected to a contact point of resistance R1and resistance R2, and the other end grounded. Further, the filtercircuit 18 includes a capacitor C2 with one end connected to a contactpoint of resistance R2 and resistance R3, and the other end grounded,and a series circuit of the resistance R4 and the capacitor C3 which isconnected between the contact point of the resistance R2 and resistanceR3 and the inverting input terminal of the operational amplifier OP1.Further, a reference voltage obtained by dividing constant voltage Vccby the series circuit of resistances R5, R6, is input into thenon-inverting input terminal of the operational amplifier OP1.

With this structure, the filter circuit 18 has a function as a low-passfilter and an inverting amplifier circuit, and when the square wavevoltage signal is input, this voltage signal is converted to thesinusoidal voltage signal with polarity inverted, which is then output.

The current control circuit 19 includes NPN-type transistor Tr1 with thecollector connected to the communication line 4 via the rectifier 11. Inthe transistor Tr1, the emitter is grounded via resistance R7, and thebase is connected to the output end of the filter circuit 18 (output endof the operational amplifier OP1). The emitter of the transistor Tr1 isconnected to the contact point of resistance R4 and capacitor C3, tothereby form a feedback path of the operational amplifier OP1. Thus, thefeedback of the operational amplifier OP1 is not directly taken from theoutput terminal of the operational amplifier OP1, but is taken from theemitter of the transistor Tr1, so that feedback can be given inconsideration of the temperature characteristic and voltage drop of thetransistor Tr1. Thus, there is an advantage that the followability ofthe current signal (primary signal 51) can be improved.

With this structure, when the transmission signal, being the square wavevoltage signal, is input into the input end 101 of the filter circuit18, the sinusoidal voltage signal appears on the output end of thefilter circuit 18 (output terminal of the operational amplifier OP1). Atthis time, in the current control circuit 19, the magnitude of thecurrent flowing through the transistor Tr1 is changed according to anoutput voltage level of the filter circuit 18, and therefore themagnitude of the current drawn into the communication terminal 10 fromthe communication line 4, is changed. As a result, the current signal(primary signal 51) corresponding to the voltage signal output by thefilter circuit 18 is generated on the communication line 4. Namely, itis the same thing that the transmitting circuit 14 generates the primarysignal 51, being the current signal, on the communication line 4 bychanging the current flowing from the communication line 4, and that thetransmitting circuit 14 transmits the primary signal 51 to thecommunication line 4 by changing the current flowing from thecommunication line 4.

The primary signal 51 thus transmitted (generated) on the communicationline 4 by the transmitting circuit 14, is converted to the voltagesignal by voltage drop by the current/voltage converter 3. In otherwords, the transmitting circuit 14 generates the change of voltage(voltage signal) on the communication line 4 by voltage drop by changingthe current flowing from the communication line 4, and such a change ofvoltage is received by the receiving circuit 15 as the secondary signal52.

Namely, when a certain square wave transmission signal is input into thefilter circuit 18 from the controller 17, the voltage signal as shown inFIG. 4B is generated on the output end of the filter circuit 18, and thecurrent flowing through the current control circuit 19 is changed. Atthis time, the primary signal 51, being the current signal, generated onthe communication line 4 is converted to the secondary signal 52 beingthe voltage signal with polarity inverted as shown in FIG. 4A, by thecurrent/voltage converter 3. Note that FIG. 4C shows the data receivedby the receiving circuit 15.

As shown in FIG. 3, the series circuit of resistance R8 and capacitor C4is connected between connection ends to the transmitting circuit 14 ofthe rectifier 11.

Subsequently, a specific structure of the receiving circuit 15 will bedescribed, with reference to FIG. 5.

The receiving circuit 15 includes an amplifier circuit 21 connected tothe output of the rectifier 11 via the capacitor C20, a comparatorcircuit 22 connected to the output of the amplifier circuit 21, and acorrection circuit 23 connected to the output of the comparator circuit22. An output end 102 of the correction circuit 23 is connected to thecontroller 17.

In an example of FIG. 5, the amplifier circuit 21 is an active filterusing an operational amplifier OP2 in which a reference voltage isapplied to the non-inverting input terminal. The amplifier circuit 21includes resistance R21 and resistance R22 connected in seriessequentially from the input end side between the input end and theinverting input terminal of the operational amplifier OP2, and capacitorC21 with one end connected to a contact point of resistance R21 andresistance R22 and the other end grounded. Further, the amplifiercircuit 21 includes resistance R23 connected between the contact pointof resistance R21 and resistance R22, and the output terminal of theoperational amplifier OP2, and a capacitor C22 connected between theinverting input terminal and the output terminal of the operationalamplifier OP2. Further, the reference voltage obtained by dividing theconstant voltage Vcc by the series circuit of resistances R24, R25, isinput into the non-inverting input terminal of the operational amplifierOP2.

With this structure, the amplifier circuit 21 has a function as theinverting amplifier circuit, and when the voltage signal is input, thevoltage signal is converted to the voltage signal with polarityinverted, which is then output.

The comparator circuit 22 includes an operational amplifier OP3 in whicha reference voltage is applied to the non-inverting input terminal. Thecomparator circuit 22 further includes resistance R26 connected betweenthe output of the amplifier circuit 21 (output terminal of theoperational amplifier OP2) and the non-inverting input terminal of theoperational amplifier OP3, and resistance R27 connected between theinverting input terminal and the output terminal of the operationalamplifier OP3. Further, the reference voltage obtained by dividing theconstant voltage Vcc by the series circuit of resistances R28, R29 isinput into the inverting input terminal of the operational amplifierOP3, and resistance R30 is connected between the constant voltage Vccand the output terminal of the operational amplifier OP3.

As will be described in detail later, the correction circuit 23 is thecircuit for outputting the received signal, being the square wavevoltage signal, to the controller 17 by correcting a signal waveformoutput from the comparator circuit 22.

With this structure, when the secondary signal 52, being the voltagesignal, is generated on the communication line 4, the voltage signal,being the signal showing the change of voltage of the secondary signal52 with a reference voltage as a reference, appears on the output end ofthe amplifier circuit 21 (output terminal of the operational amplifierOP2). At this time, the comparator circuit 22 binarizes the secondarysignal 52 by comparing the voltage signal output by the amplifiercircuit 21 and the reference voltage of the operational amplifier OP3.The secondary signal 52 thus binarized, is output to the controller 17from the output end 102 as the received signal, with its waveformcorrected by the correction circuit 23.

Namely, when the secondary signal 52 shown in FIG. 6A is input from thecommunication line 4, the output voltage of the amplifier circuit 21 ischanged according to the change of voltage of the secondary signal 52 asshown in FIG. 6B. At this time, the output voltage of the amplifiercircuit 21 is binarized by the comparator circuit 22, to generate asignal as shown in FIG. 6C in the output of the comparator circuit 22,and further by correcting the waveform by the correction circuit 23, thesquare waveform received signal as shown in FIG. 6D is output to thecontroller 17.

A baseband transmission system is used in the communication of thecommunication terminals 10. The baseband transmission system is a systemfor transmitting a pulse signal corresponding to two values of “1” and“0” as DC binary coding without modulating the signal. Further, asingle-flow RZ (return to zero) system is employed as a signal systemhere, and a combination of the “L-level” and the “H-level” in thetransmission signal and the received signal is set to “1”, and acombination of the “H-level” and the “H-level” is set to “0”.

According to the communication system of this embodiment describedabove, the signal transmitting side communication terminal 10 transmitsthe signal to the communication line 4 in the form of the primary signal51, being the current signal, and then the primary signal 51 isconverted to the voltage signal by the current/voltage converter 3.Thus, the signal receiving side communication terminal 10 receives thesecondary signal 52, being the voltage signal. Namely, although thecommunication is performed between the communication terminals 10, whenfocusing on the secondary signal 52, the communication is performedbetween the current/voltage converter (actually including thecharacteristic impedance of the communication line 4 as described above)3 which is the origin of the secondary signal 52, and the receiving sidecommunication terminal 10.

Accordingly, because of providing the current/voltage converter 3between the communication terminals 10 and 10, a communication distancebetween the communication terminals 10, 10 can be extended whilesecuring a high transmission quality. Thus, in the aforementionedcommunication system, the communication can be performed between thecommunication terminals 10 and 10 which are relatively away from eachother, by suppressing a distance between the current/voltage converter 3and the receiving side communication terminal 10 even when the apparentcommunication distance between the communication terminals 10, 10 isincreased.

When specifically described, it is confirmed that the following resultcan be obtained by using the aforementioned communication system. Here,the following case as shown in FIG. 7 is assumed. Namely, using acommunication line 4 having a total length of 1000 [m], the transmissionunit 2 is connected to the communication line 4 at a position(intermediate position) of 500 [m] from one end thereof via thecurrent/voltage converter 3, and a installation position of thereceiving side communication terminal 10 on the communication line 4 isgradually changed. Note that in an example of FIG. 7, the communicationline 4 is forked into two branches from its center position.

FIG. 8 shows the amplitudes (peak to peak) of the secondary signals 52in the receiving side communication terminal 10, in a case that thedistance from said one end of the communication line 4 to thetransmitting side communication terminal 10 is changed sequentially from0, 100, 200, 300, 400, and 500 [m]. In FIG. 8, “D1” shows an amplitudewhen the distance from said one end of the communication line 4 to thetransmitting side communication terminal 10 is 0 [m], “D2” is anamplitude when the distance is 100 [m], “D3” is an amplitude when thedistance is 200 [m], “D4” is an amplitude when the distance is 300 [m],“D5” is an amplitude when the distance is 400 [m], and “D6” is anamplitude when the distance is 500 [m]. In FIG. 8, the horizontal axisindicates the distance from said one end of the communication line 4 tothe receiving side communication terminal 10, and the vertical axisindicates the amplitude of the secondary signal 52.

As is clarified from FIG. 8, by using the communication system of thisembodiment, the receiving side communication terminal 10 can receive thesecondary signal 52 having sufficient amplitude, irrespective of theinstallation position of the communication terminal 10 on thecommunication line 4. For example, even in a case that the transmittingside communication terminal 10 is set on said one end of thecommunication line 4, the transmitting side communication terminal 10 isset on the other end of the communication line 4, and the communicationdistance is set to 1000 [m], the secondary signal 52 has the sufficientamplitude exceeding 1 [Vpp].

Further, under the same condition of FIG. 8, when the position of thetransmitting side communication terminal 10 was changed in a range of 0to 500 [m] from said one end of the communication line 4, and theposition of the receiving side communication terminal 10 was changed ina range of 0 to 1000 [m] from one end of the communication line 4, itwas confirmed that communication error was not generated at eitherposition. Specifically, it was confirmed that when transmitting the dataof the 1000 packets from the transmitting side communication terminal10, the data of all 1000 packets could be received by the receiving sidecommunication terminal 10, irrespective of the positions of thecommunication terminals 10. Thus, the communication distance between thecommunication terminals 10 can be extended while securing a hightransmission quality, by using the aforementioned communication system.

Further, FIG. 9 shows the amplitude of the secondary signal 52 receivedby the receiving side communication terminal 10, when the transmittingside communication terminal 10 is set on said one end of thecommunication line 4 and the position of the receiving sidecommunication terminal 10 is changed, in the structure of FIG. 7. Here,it shows results when the resistance values of the resistance elementsR11, R12 that constitute the current/voltage converter 3, are set to4.7Ω. Note that in FIG. 9, the horizontal axis indicates the distancefrom said one end of the communication line 4 to the receiving sidecommunication terminal 10, and the vertical axis indicates the amplitudeof the secondary signal 52, and actually measured values are shown byblack circles, and analytic values are shown by solid line.

As is clarified from FIG. 9, in the aforementioned communication system,the communication between the communication terminals 10 is enabled,even when the input impedance of the transmission unit 2 is set to below. Thus, a structure which hardly allows the noise to be generated canbe achieved by setting low input impedance of the transmission unit 2.Namely, transmission of the primary signal 51 is enabled even in a caseof low line impedance of the communication line 4, by using the currentsignal as the primary signal 51 transmitted from the communicationterminal 10.

Accordingly, when the communication terminal 10 is introduced, thecontractor is not required to perform a work of confirming the positionof the already established terminal (transmission unit 2, firstcommunication terminal 1) when connecting a high impedance module, andit is sufficient to simply connect the communication terminal 10 to thecommunication line 4. Therefore, the load of an installation operationof the communication terminal 10 can be tremendously reduced.

As described above, according to the aforementioned communicationsystem, there is an advantage that labor of examining a situation of awork site and confirming the communication distance can be reduced, thusreducing the load of the contractor, and introduction of thecommunication system can be facilitated.

Incidentally, in the communication system with the aforementionedstructure, in a case of a long communication line 4, the amplitude ofthe voltage signal (secondary signal 52) generated on the communicationline 4 becomes large, and accordingly a transient response generated byRC component on the communication line 4 becomes also large. Therefore,there is a possibility that communication error is generated due to thevoltage change caused by the transient response.

For example, when the square wave transmission signal shown in FIG. 10Cis input into the transmitting circuit 14 from the controller 17, thecurrent flowing to the transmitting circuit (current control circuit 19)14 is changed as shown in FIG. 10B. At this time, the current signal(primary signal 51) generated on the communication line 4 is convertedto the voltage signal by the current/voltage converter 3, to therebygenerate the change of voltage on the communication line 4 as shown inFIG. 10A. Therefore, the receiving side communication terminal 10receives the secondary signal 52, being the voltage signal shown in FIG.10A. However, since the amplitude of the secondary signal 52 is large,in the receiving circuit 15, an unwanted component (transient responsecomponent) which is not the signal component corresponding to thetransmission signal also exceeds the reference voltage (threshold value)of the comparator circuit 22, and the received signal including theunwanted component as shown in FIG. 10D is obtained. After all, thereceived signal includes the unwanted component due to the change ofvoltage caused by the transient response, and therefore the receivedsignal does not become identical to the transmitting signal, thusgenerating the communication error. Note that FIG. 10B shows the outputvoltage of the operational amplifier OP1 that reflects the currentflowing through the transistor Tr1.

Therefore, in this embodiment, the communication terminal 10 has astructure of reducing the input impedance viewed from the communicationline 4 when transmitting the primary signal 51 to the communication line4 from the transmitting circuit 14, thereby prevents the amplitude ofthe secondary signal 52 generated on the communication line 4 frombecoming unnecessarily large. Specifically, the series circuit ofresistance R8 and capacitor C4 connected between connection ends for thetransmitting circuit 14 of the rectifier 11 is configured so that theycan be separated from each other, and the series circuit is connectedonly when the primary signal 51 is transmitted from the transmittingcircuit 14, to thereby reduce the impedance compared with the impedanceof other period. During reception by the receiving circuit 14, for thepurpose that the secondary signal 52 generated on the communication line4 can be received, the series circuit is separated so as to have apredetermined impedance to the secondary signal 52. Switching of theimpedance is realized using a semiconductor switch, etc., that can becontrolled by the controller 17.

As described above, by reducing the input impedance of the communicationterminal 10 when the primary signal 51 is transmitted, the amplitude ofthe secondary signal 52 does not become unnecessarily large as shown inFIG. 11A, and the change of voltage by transient response can besuppressed to be small. As a result, in the receiving side communicationterminal 10, as shown in FIG. 11D, the received signal not including theunwanted component due to the transient response can be output to thecontroller 17 from the receiving circuit 15, and therefore thecommunication error can be reduced. The transmitting signal is shown inFIG. 11C, and the output voltage of the operational amplifier OP1reflecting the current flowing through the transmitting circuit 14 isshown in FIG. 11B.

Further, in the receiving circuit 15, the reference voltage (firstreference voltage) of the amplifier circuit 21 is set to be deviatedfrom the reference voltage (second reference voltage) of the comparatorcircuit 22, so that the unwanted component due to the change of voltageby transient response is not included in the received signal.

Namely, the voltage signal changed according to the change of voltage ofthe secondary signal 52 with the reference voltage as a reference, isoutput from the amplifier circuit 21. Therefore, if the referencevoltage of the amplifier circuit 21 and the reference voltage of thecomparator circuit 22 are set to the same values, there is a possibilitythat the unwanted component due to transient response is also includedin the received signal. For example, when the square wave transmissionsignal shown in FIG. 12D is input into the transmitting circuit 14 fromthe controller 17, the current signal (primary signal 51) generated onthe communication line 4, is converted to the voltage signal by thecurrent/voltage converter 3, thus generating the change of voltage onthe communication line 4 as shown in FIG. 12A. At this time, in thereceiving side communication terminal 10, the secondary signal 52, beingthe voltage signal shown in FIG. 12A is received, and as shown in FIG.12B, the output of the amplifier circuit 21 is changed according to thechange of voltage of the secondary signal 52, with the reference voltageVs0 of the comparator circuit 22 as a reference. Therefore, the receivedsignal output to the controller 17 from the receiving circuit 15includes the unwanted component due to the change of voltage caused bytransient response as shown in FIG. 12C, and therefore the receivedsignal does not correspond to the transmitting signal, resulting in thecommunication error.

Meanwhile, in this embodiment, by differentiating the resistance valuesof resistance R24 and resistance R28 of the receiving circuit 15, thereference voltage of the amplifier 21 is set to be lower than thereference voltage of the comparator circuit 22. Thus, as shown in FIG.13B, the output of the amplifier circuit 21 is changed according to thechange of voltage of the secondary signal 52, with reference voltage Vs1as a reference, which is lower than the reference voltage Vs2 of thecomparator circuit. Therefore, in the comparator circuit 22, theunwanted component caused by transient response can be removed, and asshown in FIG. 13C, the unwanted component caused by transient responsecan be prevented from being included in the received signal output tothe controller 17 from the receiving circuit 15, and therefore thecommunication error can be reduced. The transmitting signal is shown inFIG. 13D, and the secondary signal 52 is shown in FIG. 13A.

A function of the correction circuit 23 provided in the receivingcircuit 15 will be described next.

Namely, in the receiving circuit 15, the secondary signal 52 issometimes received in such a manner as being deviated from a regularwaveform caused by distortion of current waves and a phase lag, whichresults in deterioration of the transmission quality. Therefore, in thisembodiment, the communication error is reduced by correcting a timewidth (pulse width) of the signal output from the comparator circuit 22,to a fixed value by the correction circuit 23.

The correction circuit 23 is configured to detect the rising or fallingof the secondary signal 52 as a trigger, and correct the secondarysignal 52 in a square wave having a predetermined pulse width from adetection time point of the trigger. The pulse width of the signal aftercorrection, is determined using a timer function (one shot timer) of amicrocomputer of the controller 17 for example.

However, the transient response is generated relatively large at a startof the flow of the current to the transmitting circuit 14, and thereforethe pulse width of a leading pulse is sometimes larger than thefollowing pulse, in the output of the amplifier circuit 21. Namely, thepulse width of the output of the amplifier circuit 21 is not constant asshown in FIG. 13B, and therefore variation is sometimes generated in thepulse width in the signal after correction as shown in FIG. 13C, byusing the falling of the secondary signal 52 (rising of the output ofthe amplifier circuit 21) as a trigger. In an example of FIG. 13C, timewidth w2 of the leading 1 bit showing the value of “1” is larger thantime width w1 of the following 1 bit.

Therefore, preferably the correction circuit 23 employs a structure ofdetecting the rising of the secondary signal 52 (falling of the outputof the amplifier circuit 21) as a trigger. Thus, even when there is avariation in the pulse width of the output of the amplifier circuit 21as shown in FIG. 14B, time width w1 of 1 bit can be fixed to anapproximately constant value in the signal after correction as shown inFIG. 14C. The transmission signal is shown in FIG. 14D, and thesecondary signal 52 is shown in FIG. 14A.

Thus, even in a case that distortion is generated in the secondarysignal 52 in a time axial direction under an influence of transientresponse, the communication error can be suppressed by correcting thisdistortion, in such a way that the secondary signal 52 is corrected tothe square wave with a predetermined pulse width.

Note that according to this embodiment, in the transmitting circuit 14,the current signal (primary signal 51) of the transmitting signal, withpolarity inverted, output from the controller 17, is drawn into thecommunication terminal 10 from the communication line 4 by using thefilter circuit 18 having a function as an inverting amplifier circuit.However, the embodiment is not limited thereto, and for example thecurrent signal (primary signal 51) with the same polarity as thepolarity of the transmission signal output from the controller 17, mayalso be drawn into the communication terminal 10 from the communicationline 4, by using the filter circuit 18 as the non-inverting amplifiercircuit for example. In this case, a direction of the change of voltagegenerated by the change of current of the primary signal 51 is reversedby the current/voltage converter 3. Therefore, the voltage generatedbetween communication lines 4 is increased when transmitting the primarysignal 51. With this structure, although power consumption is increasedduring standby, compared with the aforementioned embodiment, the powerconsumption during transmitting the signal can be reduced. Further,similar operation can be obtained even if the polarity itself of thetransmission signal output from the controller 17 is inverted, withoutchanging the structure of the transmitting circuit 14.

Further, specific circuits such as filter circuit 18, amplifier circuit21, and comparator circuit 22 shown in FIG. 3 and FIG. 5 are simplyexamples, and an equivalent function can be realized by other circuitstructure as well.

Embodiment 2

The communication system of this embodiment is different from thecommunication system of Embodiment 1, in a point that an encodingsection (not shown) for encoding transmission data received/transmittedbetween the communication terminals 10 is provided in the transmittingcircuit 14, and a decoding section (not shown) for decoding data andacquiring the transmission data is provided in the receiving circuit 15.

Namely, the receiving circuit 15 detects the rising of the secondarysignal 52 by the correction circuit 23 as a trigger, and corrects thepulse width. Therefore, in the receiving circuit 15, when the value (“0”here) not generating the rising in the secondary signal 52 is continued,the trigger is not detected in the correction circuit 23, which mayresult in the deterioration of the transmission quality. Therefore, inthis embodiment, continuance of value “0” in the secondary signal 52 isprevented by encoding the transmission data by the encoding section.Note that value “1” generates rising, and therefore there is noinfluence on the transmission quality even if the value “1” iscontinued.

When specifically explained, the encoding section divides thetransmission data into data sequences for each of the 5 bits, andconverts each data sequence to the data sequence of 8 bits in whichvalue “0” is not continued. A table used for converting the datasequence as shown in table 1 is previously registered in thecommunication terminal 10, and the encoding section converts the datasequence using this table. The data sequence of 8 bits (“1” to “32” oftable 1) assigned to each data sequence of 5 bits (there are 32different data sequences) is stored in the table. Further, a start codeshowing a start of transmitting data (“Start code” of table 1) STX and astop code showing an end of transmitting data (“End code” of table 1)ETX are stored in the table.

Namely, both values of “0xFD” and “0xFF” are used not as data but as thestart code STX and the stop code ETX of a specific code, so as to obtaina clear division of packets. Further, “0” as start bit S, and “1” asstop bit E, are respectively assigned to before and after each datasequence of 8 bits. Accordingly, finally, data sequence of 5 bits asshown in FIG. 15A is converted to the data sequence of 10 bits as shownin FIG. 15B respectively. Note that in table 1, the data sequence afterconversion is also expressed by hexadecimal notation.

TABLE 1 Before After conversion conversion S 0 1 2 3 4 5 6 7 EHexadecimal  1 0 0 0 0 0 0 1 0 1 0 1 0 1 0 1 0x55  2 0 0 0 0 1 0 1 1 1 01 0 1 0 1 0x57  3 0 0 0 1 0 0 1 1 0 1 1 0 1 0 1 0x5B  4 0 0 0 1 1 0 1 01 1 1 0 1 0 1 0x5D  5 0 0 1 0 0 0 1 1 1 1 1 0 1 0 1 0x5F  6 0 0 1 0 1 01 1 0 1 0 1 1 0 1 0x6B  7 0 0 1 1 0 0 1 0 1 1 0 1 1 0 1 0x6D  8 0 0 1 11 0 1 1 1 1 0 1 1 0 1 0x6F  9 0 1 0 0 0 0 1 0 1 0 1 1 1 0 1 0x75 10 0 10 0 1 0 1 1 1 0 1 1 1 0 1 0x77 11 0 1 0 1 0 0 1 1 0 1 1 1 1 0 1 0x7B 120 1 0 1 1 0 1 0 1 1 1 1 1 0 1 0x7D 13 0 1 1 0 0 0 1 1 1 1 1 1 1 0 1 0x7F14 0 1 1 0 1 0 1 1 0 1 0 1 0 1 1 0xAB 15 0 1 1 1 0 0 1 0 1 1 0 1 0 1 10xAD 16 0 1 1 1 1 0 1 1 1 1 0 1 0 1 1 0xAF 17 1 0 0 0 0 0 1 0 1 0 1 1 01 1 0xB5 18 1 0 0 0 1 0 1 1 1 0 1 1 0 1 1 0xB7 19 1 0 0 1 0 0 1 1 0 1 11 0 1 1 0xBB 20 1 0 0 1 1 0 1 0 1 1 1 1 0 1 1 0xBD 21 1 0 1 0 0 0 1 1 11 1 1 0 1 1 0xBF 22 1 0 1 0 1 0 1 0 1 0 1 0 1 1 1 0xD5 23 1 0 1 1 0 0 11 1 0 1 0 1 1 1 0xD7 24 1 0 1 1 1 0 1 1 0 1 1 0 1 1 1 0xDB 25 1 1 0 0 00 1 0 1 1 1 0 1 1 1 0xDD 26 1 1 0 0 1 0 1 1 1 1 1 0 1 1 1 0xDF 27 1 1 01 0 0 1 1 0 1 0 1 1 1 1 0xEB 28 1 1 0 1 1 0 1 0 1 1 0 1 1 1 1 0xED 29 11 1 0 0 0 1 1 1 1 0 1 1 1 1 0xEF 30 1 1 1 0 1 0 1 0 1 0 1 1 1 1 1 0xF531 1 1 1 1 0 0 1 1 1 0 1 1 1 1 1 0xF7 32 1 1 1 1 1 0 1 1 0 1 1 1 1 1 10xFB Start cord 0 1 0 1 1 1 1 1 1 1 0xFD End cord 0 1 1 1 1 1 1 1 1 10xFF

The decoding section decodes the data sequence of 8 bits converted bythe encoding section based on the table of table 1, by inverselyconverting it to the data sequence of 5 bits. Thus, the transmissiondata before encoding can be acquired by the receiving circuit 15, fromthe encoded data.

When a specific example is given, as shown in FIG. 16A, when thetransmission data of 5 bytes (40 bits) is transmitted as shown in FIG.16A, first, the encoding section divides the transmission data intoeight data sequences of 5 bits unit as shown in FIG. 16B. Subsequently,the encoding section converts each data sequence to the data sequence of8 bits based on the table of table 1. Further, the encoding sectionconverts each data sequence to ten data sequences of 8 bits in total (10bits including start bit and stop bit) as shown in FIG. 16C, by addingstart code STX and stop code ETX to before and after transmission data.Therefore, when the transmission data of 5 bytes is transmitted during 1ms, data of 100 bits in total (=10 bits×10) is transmitted.

When a data transmitting speed of UART (Universal Asynchronous ReceiverTransmitter) is set to 156.25 kbps, 6.4 μs is required for transmittingdata of 1 bit, and therefore 640 μs is required for transmitting data of100 bits. Note that the communication suitable period of 1 ms per 1period (15 ms) exists at three places in the transmission signal usedfor the communication between the transmission unit 2 and the firstcommunication terminals 1. Therefore, when the transmission data of 5bytes is transmitted with no conversion, the data transmitting speed of800 bps (=3/15 [ms]×40 [bits]) is sufficient.

According to the structure of this embodiment described above, the valuenot generating the trigger in the secondary signal 52 is not continuedby encoding the transmission data, and the encoded transmission data istransmitted. Accordingly, there is an advantage that the secondarysignal 52 can be surely corrected by the correction circuit 23, and thereliability of the communication can be improved.

A specific operation of this embodiment will be described. For example,when data of “1101011” is transmitted, regularly the receiving circuit15 receives the secondary signal 52 shown in FIG. 17A, and generates thesignal as shown in FIG. 17B in the output of the comparator circuit 22.Thus, the signal after being corrected by the correction circuit 23becomes the signal corresponding to the transmission data “1101011” asshown in FIG. 17C.

Further, when the distortion is generated in the secondary signal 52 asshown in FIG. 18A, the receiving circuit 15 sometimes generates thesignal with no constant pulse width as shown in FIG. 18B, in the outputof the comparator circuit 22. In this case as well, the signal afterbeing corrected by the correction circuit 23, becomes the signalcorresponding to the transmission data “1101011” as shown in FIG. 18C.

Meanwhile, if data of “1100011” is transmitted and the secondary signal52 is received wherein the value (“0” here) not generating rising asshown in FIG. 19A is continued, the receiving circuit 15 generates thesignal as shown in FIG. 19B in the output of the comparator circuit 22.In this case, the trigger is effected only in the last “0” of thecorrection circuit 23 in a part where “0” is continued in the output ofthe comparator circuit 22, and therefore the signal after correctionshows “1111011” as shown in FIG. 19C, which is different from thetransmission data “1100011”, and the communication error is therebycaused.

In the structure of this embodiment, by encoding the transmission dataand transmitting the encoded transmission data, there is no continuanceof the value not generating the trigger in the secondary signal 52.Therefore, the communication error as shown in FIG. 19C can be avoided.

The other structure and function are similar to those of Embodiment 1.

Incidentally, the aforementioned each embodiment shows an example ofapplying voltage to the communication line 4 by transmitting thetransmitting signal, being the voltage signal, by the transmission unit2. However, the embodiment is not limited thereto, and a power supplydevice for applying constant DC voltage to the communication line 4 maybe provided instead of the transmission unit 2. In this case, thecommunication terminal 10 performs communication with othercommunication terminal 10 by transmitting, to the communication line 4,the packet including data to be transmitted to other communicationterminal 10, by superimposing it on the DC voltage based on the secondprotocol. In this case, the data transmitting speed of UART is 78.125kbps, which is half of the aforementioned 156.25 kbps.

Further, the current/voltage converter 3 is not limited to have astructure composed of resistance elements R11, R12 inserted between theoutput end of the transmission unit 2 and the communication line 4 (seeFIG. 3), but may have a structure composed of an impedance upper sectionusing a transformer as shown in FIG. 20 for example.

In an example of FIG. 20, the current/voltage converter 3 includessecondary winding inductors (transformers) M1, M2 consisting of primaryand secondary coils, and diodes D1 to D8, in addition to resistanceelements R101, R102. The primary and secondary coils of the secondarywinding inductors M1, M2 function as inductance elements respectively,and are utilized as resistance components of the current/voltageconverter 3.

Specifically, in parallel to the resistance element R101 inserted to oneof the two-wire communication lines 4, the series circuit of diodes D1,D5, and the series circuit of diodes D2, D6 are connected in reverseparallel to each other. In the secondary winding inductor M1, theprimary coil is connected between both ends of diode D2, and thesecondary coil is connected between both ends of diode D1.

Similarly, in parallel to the resistance element R102 inserted to theother of the two-wire communication lines 4, the series circuit ofdiodes D3, D7, and the series circuit of diodes D4, D8 are connected inreverse parallel to each other. In the secondary winding inductor M2,the primary coil is connected between both ends of diode D3, and thesecondary coil is connected between both ends of diode D4.

According to this structure, by using inductances of the secondarywinding inductors M1, M2, the current/voltage converter 3 shows lowresistance to the DC current component, while showing sufficientresistive impedance to the signal component used for communication.Namely, when the resistance values of the resistance elements R101, R102are set to 4.7Ω for example, the input impedance of the current/voltageconverter 3 is about 10Ω in a frequency band of the signal component(for example 78 kHz) used for communication. Meanwhile, DC resistance ofthe current/voltage converter 3 is determined by a coil resistance (forexample, about 2Ω) of the secondary winding inductors M1, M2, andtherefore it becomes low resistance.

As a result, the DC resistance can be set to be small in thecurrent/voltage converter 3 having the structure shown in FIG. 20,compared with that of the current/voltage converter 3 composed ofresistance elements R11, R12 (see FIG. 3), while equalizing theimpedance to the signal component. Therefore, there is an advantage thatsufficient voltage can be applied to an end terminal, even if thecurrent supplied from the transmission unit 2 is increased, therebyincreasing the voltage drop in the current/voltage converter 3. Forexample, when the current of 500 mA flows with line resistance 16Ω ofthe communication line 4, the voltage drop of about 13 V occurs on thelast end if the resistance of the current/voltage converter 3 is 10Ω.However, if the resistance of the current/voltage converter 3 is 2Ω, thevoltage drop is suppressed to about 9 V even on the last end.

In addition, in the current/voltage converter 3 having the structureshown in FIG. 20, the current flows through a coil different in eachpolarity of the signal, out of the primary and secondary coils of thesecondary winding inductors M1, M2, and therefore acounter-electromotive force is small when the polarity of the signal isinverted. Accordingly, there is an advantage that the distortion of thewaveform generated on the communication line 4 when the polarity of thesignal is inverted, can be suppressed while increasing the inputimpedance by the inductance element, and the deterioration of thecommunication quality can be suppressed.

Further, FIGS. 21 to 24 show the distance between the transmitting sidecommunication terminal 10 and the receiving side communication terminal10, and the waveform obtained in each section of the receiving sidecommunication terminal 10 when the number of the communication terminals10 connected to the communication line 4 is changed, in the structure ofthe aforementioned each embodiment. In FIGS. 21 to 24, “A” shows thesecondary signal 52 received by the communication terminal 10, “B” showsthe output of the amplifier circuit 21, “C” shows the output of thecomparator circuit 22, and “D” shows the output (received data) of thecorrection circuit 23.

As is clarified from the FIGS. 21 to 24, in the communication systemwith the aforementioned structure, the receiving side communicationterminal 10 can regularly receive the received data, even when thedistance between the communication terminals 10 and the number of thecommunication terminals 10 are changed, and even when the distortion ofthe waveform and attenuation of the signal are generated in thesecondary signal 52.

The invention claimed is:
 1. A communication system comprising: aplurality of communication terminals connected to a two-wirecommunication line; a power supply that applies a voltage between twowires of the communication line; and a current/voltage converter thatconverts a current change on the communication line to a voltage changeon the communication line by voltage drop by two resistance components,each of the plurality of communication terminals comprising: atransmitting circuit that is connected to the communication line andgenerates a primary current signal on the communication line, bychanging a current flowing from the communication line; and a receivingcircuit that receives a secondary signal, which is generated byconverting the primary current signal to a voltage signal by thecurrent/voltage converter, wherein the current/voltage convertercomprises the two resistance components, each being positioned along oneof the two wires, and connected in parallel.
 2. The communication systemaccording to claim 1, wherein a characteristic impedance of thecommunication line is utilized as each of the resistance components bythe current/voltage converter.
 3. The communication system according toclaim 1, wherein each of the resistance components comprises aresistance element connected between the power supply and thecommunication line.
 4. The communication system according to claim 1,wherein each of the resistance components comprises an inductanceelement connected between the power supply and the communication line.5. The communication system according claim 1, wherein the receivingcircuit receives, as the secondary signal, the voltage change which isgenerated on the communication line due to the voltage drop when thecurrent flowing from the communication line is changed by thetransmitting circuit.
 6. The communication system according to claim 1,wherein the power supply comprises a signal transmitter that transmits afirst protocol voltage signal to the communication line by changing amagnitude of a voltage applied to the communication line, and thereceiving circuit receives, as the secondary signal, a secondaryprotocol signal having a higher frequency than a frequency of the firstprotocol voltage signal and superimposed on the first protocol voltagesignal.
 7. The communication system according to claim 1, wherein thereceiving circuit comprises a comparator circuit that extracts a signalcomponent of the secondary signal, by binarizing the secondary signal bycomparing the magnitude of the secondary signal with a predeterminedthreshold value, and when transmitting the primary signal from thetransmitting circuit, the communication terminal reduces an inputimpedance viewed from the communication line to suppress an amplitude ofthe secondary signal such that a component other than the signalcomponent of the secondary signal does not exceed the threshold value.8. The communication system according to claim 1, wherein the receivingcircuit comprises: an amplifier circuit that converts the secondarysignal to the voltage change from a first reference voltage, byamplifying the secondary signal; and a comparator circuit that binarizesthe secondary signal, by comparing an output of the amplifier circuitwith a secondary reference voltage, the first reference voltage beingdifferent from the second reference voltage.
 9. The communication systemaccording to claim 1, wherein the receiving circuit comprises: acomparator circuit that binarizes the secondary signal, by comparing amagnitude of the secondary signal with a predetermined threshold value;and a correction circuit that detects rising or falling of the secondarysignal as a trigger, and corrects the secondary signal to a square wavehaving a predetermined pulse width from a detection point of thetrigger.
 10. The communication system according to claim 9, wherein thetransmitting circuit includes an encoder that encodes transmission datasuch that a value that does not generate the trigger does not continuein the secondary signal, and the receiving circuit includes a decoderthat decodes the encoded data obtained from the secondary signal toacquire the transmission data.
 11. The communication system according toclaim 1, wherein the plurality of communication terminals comprise atransmission side communication terminal and a receiving sidecommunication terminal to communicate each other via the current/voltageconverter, the current/voltage converter converts the current changebased on the primary current signal generated by the transmittingcircuit of the transmission side communication terminal, to the voltagechange to generate the secondary signal, and the receiving circuit ofthe receiving side communication terminal receives the secondary signalgenerated by the current/voltage converter.
 12. The communication systemaccording to claim 1, wherein the current/voltage converter isconfigured to convert the primary current signal generated in thetransmitting circuit into the secondary signal by voltage drop by thetwo resistance components, and transmit the secondary signal to thecommunication line.